This is an open-label, multicenter, two-arm Phase II clinical trial that will evaluate the impact of 2nd line chemotherapy (i.e. capecitabine) on survival in patients with non-Luminal A hormone receptor-positive (HR+) metastatic breast cancer (MBC)

This phase II trial tests how well evaluating circulating tumor deoxyribonucleic acid (ctDNA) works to guide therapy-change decisions in treating patients with triple-negative breast cancer (TNBC) that has spread from where it first started (primary site) to other places in the body (metastatic). This study wants to learn if small pieces of DNA associated with a tumor (called circulating tumor DNA, or ctDNA) can be detected in investigational blood tests during the course of standard chemotherapy treatment for breast cancer, and whether information from such investigational ctDNA blood testing could possibly be used as an early indication of chemotherapy treatment failure. It is hoped that additional information from investigational blood testing for ctDNA could help doctors to switch more quickly from a standard chemotherapy treatment that typically has significant side effects and which may not be working, to a different standard treatment regimen against TNBC, called sacituzumab govitecan. Sacituzumab govitecan is a monoclonal antibody, called hRS7, linked to a chemotherapy drug, called irinotecan. hRS7 is a form of targeted therapy because it attaches to specific molecules (receptors) on the surface of cancer cells, known as TROP2 receptors, and delivers irinotecan to kill them. Studying ctDNA may assist doctors to change therapy earlier if needed, and may improve health outcomes in patients with metastatic TNBC.

This research study is studying a combination of HER2-directed therapies (trastuzumab and pertuzumab) and hormonal therapy as a treatment after surgery for hormone receptor positive breast cancer.

The study drugs involved in this study are:

* A combination of trastuzumab and pertuzumab given as an injection under the skin (PHESGO)
* Hormonal (endocrine) Treatment

This is a Phase 1b open-label, 2-part study in 3 treatment groups. The 3 treatment groups are as follows:

Treatment Group 1: Palazestrant (OP-1250) in combination with ribociclib (KISQALI, Novartis Pharmaceuticals Corporation).

Treatment Group 2: Palazestrant (OP-1250) in combination with alpelisib (PIQRAY, Novartis Pharmaceuticals Corporation).

Treatment Group 3: Palazestrant (OP-1250) in combination with everolimus.

Treatment Group 4: Palazestrant (OP-1250) in combination with atirmociclib.